Be careful with distinguish cultures by myths - they are usually too easy to get. If you had two cultures that require Jade in vicinty and Myth of the Panda, and the other Jade as well and Myth of the Peacock, then it is more then likely you will have them both in your Jade city. Especially if you still want to change it that you get myths by simply fighting an animal instead of subdue it.
What about more complex restrictions like:
- Killing XXX units [of a type] (iE Spartans)
- Must be a conquered culture (East Germany)
- Must be / have a Vasall
- Build / have XXX Units of a type (iE Ships for England)
- At least XXX cities (russia maybe)
- At least a pop of 50 (highly populated city states)
- Income / tourism / crime etc of XXX (Dubai (income/tourism), an anarcy state for crime) etc.
IMHO, cultures should be a) rare and / or b) develope to give them more meaning. We have like what... 50-100 cultures? And how many more are planned? Mostly, you have less then 10 or even 5 leading civs that grab most of the cultures and hoart them. At one point you just build cultures just because you can't. Not much immersion. At least that's my impression.